This invention relates to a control apparatus for an automatic transmission and, more particularly, to control at kickdown.
In general, a hydraulic pressure control circuit used in an automatic transmission is provided with a plurality of shift valves conforming to the number of transmission speeds. In order to perform an automatic shift that conforms to vehicle velocity and the opening of a throttle valve (which controls the amount of intake air), an automatic transmission of this kind is further provided with a valve (referred to as a "governor valve") which generates pressure (referred to as "governor pressure") corresponding to vehicle velocity, and a valve (referred to as a "throttle valve") for generating pressure (referred to as "throttle pressure" corresponding to the engine load (which, for example, is judged based on the opening of the throttle valve). The governor pressure and throttle pressure, which vary depending upon the vehicle velocity and load, act upon the abovementioned shift valves, and it is so arranged that the operating points of these shift valves are decided in dependence upon the equilibrium point of the two pressures, as a result of which a speed change (shift) is performed automatically in conformity with vehicle velocity and throttle valve opening.
In addition to the foregoing, an automatic transmission is also required to exercise control in such a manner that when the driver steps down on the accelerator pedal by a large amount to perform a so-called "kickdown operation", a downshift will be made to low gear to cope with the high load required. Then, if the vehicle velocity rises after the downshift, control is performed so as to make an upshift to high gear.
An example of an automatic transmission which makes such a kickdown operation possible is disclosed in the specification of Japanese Patent Publication (KOKOKU) No. 63-46302. In this disclosed automatic transmission, a kickdown valve is provided and is adapted to generate kickdown pressure. By introducing this kickdown pressure to a shift valve, the latter is changed over to the downshift side. More specifically, kickdown pressure is superimposed on throttle pressure within the shift valve. Since the equilibrium point of this superimposed pressure and the aforementioned governor pressure moves, the operating point of the shift valve changes. The operating point of this shift valve causes the shift point to move toward the high velocity side, i.e., causes the downshift to low gear to take place automatically.
In such an automatic transmission so constructed that kickdown pressure is introduced to a shift valve, not only does the shift point in the downshift move toward the side of higher vehicle velocity but so does the shift point at which the upshift is performed after the downshift.
In order that parts may be used universally, the same transmission is sometimes employed in automobiles of different types. When such is the case and the same transmission is installed in an automobile of a sportier type, for example, it is generally required that the shift point be set on the side of higher vehicle velocity for purposes of tuning. Accordingly, when an automatic transmission equipped with a kickdown valve is installed in a sporty-type automobile and the aforementioned tuning is carried out, a problem is encountered. Specifically, as shown in FIG. 1, assume that a 2.fwdarw.1 shift line of an ordinary automobile, indicated by the dashed line, is moved to the side of higher vehicle velocity, which is indicated by the alternate long and two short dashed line, as required by the specifications of a sports car. When this is done, the upshift line for a 1.fwdarw.2 upshift following the downshift also moves to the side of higher vehicle velocity, as illustrated in FIG. 1. In actuality, when the driver performs the kickdown operation, first a downshift is made from 2nd speed to 1st speed. Then, as vehicle velocity subsequently rises, an upshift from low to high gear is carried out in accordance with the 1.fwdarw.2 shift line of FIG. 1. On the other hand, all engines possess an overrun vehicle velocity, and it is undesirable to rotate the engine at an rpm that exceeds this velocity. The overrun vehicle velocity is not changed by the aforementioned tuning. Accordingly, there are cases where tuning inconveniently causes the 1.fwdarw.2 upshift line to become situated on the side of vehicle velocity higher than the overrun velocity. As a result, there are instances where, because of tuning, the changeover to the high speed side of the transmission gears is delayed, thereby causing the engine to overrun.
Measures can be taken to solve this problem. Specifically, even if the timing at which an upshift is performed following a downshift happens to move to the side of higher vehicle velocity, the action of kickdown pressure can be cancelled by exercising control to forcibly turn off the kickdown valve at the moment actual vehicle velocity reaches a predetermined vehicle velocity prior to a velocity at which engine overrun would occur. If this is done, the shift valve for low gear will change over to the upshift side at the moment the action of kickdown pressure is cancelled. Overrun can be avoided as a result.
However, this expedient involves certain drawbacks. Specifically, the kickdown pressure produced by the kickdown valve is introduced commonly to each of the shift valves which perform the 1-2 speed changeover and 2-3 speed changeover. In an automatic transmission which is so designed that the kickdown valve is forcibly turned off at vehicle velocities above a predetermined value, as in the above-described expedient, the kickdown valve will not operate in a case where the driver performs the kickdown operation while the vehicle is traveling in third or fourth gear at a velocity above the predetermined value. As a consequence, the shift valve for high gear will not change over and kickdown will not take place.